2183
Inorg. Chem. 1990, 29, 2183-2185 a crystalline precipitate of presumably B,H,.N(CH,),.) The intensity ratio for B2H4.2N(CH3),:BH,.N(CH,), remained 2:l. When the solution was allowed to staad at room temperature for 2.5 h, the signals of B4H8-N(CH3),had grown in the spectrum and the intensity of the BH,.N(CH3), signal had become greater than that of the B2H,.2N(CH,), signal. The signal of B,H,.N(CH,), still remained strong in the spectrum. Acknowledgment. We gratefully acknowledge the support of this work by the US. Army Research Office through Grant DAAG 29-85-K-0034. Supplementary Material Available: Two series of spectra for the reaction solutions of B3H7-S(CH3)2+ N(CH,), in S(CH3)2and of B,H,.THF + N(CH,), in THF (2 pages). Ordering information is given on any current masthead page.
a a5 6
t AH8
7 GH8
7.91 7.8 7.7
Contribution from the Faculty of Pharmaceutical Sciences, Nagoya City University, Tanabe-dori, Mizuho-ku, Nagoya 467, Japan, and Department of Chemistry, Gorlaeus Laboratories, Leiden University, 2300 RA Leiden, The Netherlands
Unusual Isomerization Reaction of a Platinum-Dinucleotide Compound Kenji Inagaki,**laMaarten Alink,la Akinori Nagai,Ia Yoshinori Kidani,*Ja and Jan ReedijkIb Received August 7, I989
It has been generally believed that bifunctional attack of antitumor platinum complex, e.g., C ~ S - P ~ ( N H , ) ~on C I DNA ~, is responsible for an appearance of the biological activities? Much attention has been focused on characterization of platinumoligonucleotide adducts3 formed after chelation due to cis-Pt(NHJ2Cl2. The reaction between a bifunctional platinum complex and nucleic acid base occurs via a two-step mechanism: and the first step is normally a preferential binding to the N7 site of a guaninea5 The present paper mainly describes results obtained about the second step, i.e., chelation after the first platinum binding. The reaction of [Pt,(R,R-da~h)(0H,)~]~+ with r(GpA) (abbreviations: R,R-dach, lR,2R-~yclohexanediamine;r(GpA), guanylyl(3'-5')adenosine) results in interbase cross-linked compounds between the guanine and the adenine bases. These chelate compounds are formed via the 1:l aqua intermediate, Pt(R,Rdach)(OH2)(r(GpA)-N7(I)).This process is a normal two-step reaction. An intriguing observation is that one of the chelate compounds, which is likely to be a kinetically preferred species, isomerizes very slowly to give the end products. That is, the
7.6:
AH2
; 3 k '5
6
;. 'e
9
ib i'l i:
PH Figure 1. Chemical shift vs pH of the purine base protons of the Pt The numbers along the adduct 4, Pt(R,R-dach)(r(GpA)-N7(I),NI(2)). plots indicate the line widths of the signals at half-height (Hz).
isomerization process is a reaction in which the kinetically preferred species is slowly converted to thermodynamically more stable chelates. The aim of the present paper is to introduce such an unusual isomerization reaction. Such an isomerization reaction could not be observed in the case of reaction between r(ApG) and [P t ( R , R - d a ~ h ) ( 0 H ~ ) ~ ] ~ + . The reaction of [Pt(R,R-dach)(OH2)J2+ with r(GpA) was followed by means of HPLC as a function of time (10 OC, pH 4.4). The reaction results in four Pt adducts (1-4)6 formed via an aqua 1:l intermediate. The reaction of Pt(R,R-dach)C12 with r(GpA) also results in the same four Pt adducts, in this case without an observable intermediate because the hydrolysis reaction of the dichloro complex is the rate-determining step. The relative ratio of the four Pt adducts was found to be constant throughout the reaction' because the isomerization under consideration is very slow at 10 OC. With the reaction at higher temperature, e.g. 37 OC, the relative ratio was almost constant for the first 10 h (see Table I). However, the amount of Pt adduct 4 appeared to decrease thereafter, apparently because of an isomerization reaction. The kinetic parameters obtained under the pseudofirst-order conditions [the concentrations of [Pt(R,R-dach)(OH2)2]2+were 10-40 times in excess over those of r(GpA)] are indicated in Table I. The occurring reaction can be described according to the following scheme: r(GpA) Pt(R,R-dach) 1:l aqua intermediate 1-4
+
-+
kl
kc
(1) ( I ) (a) Nagoya City University. (b) Leiden University. (2) (a) Recdijk, J.; Fichtinger-Schepman,A. M. J.; van Oosterom, A. T.; van de Putte, P.Srwcr. Bonding 1987,67,53-89 and references therein. (b) Sherman, S.E.; Lipprd, S.J. Chem. Rev. 1987,87,1153-1 181 and references therein. (3) (a) Chottard, J. C.; Girault, J. P.;Chottard, G.;Lallemand, J. Y.; Mansuy, D. J. Am. Chem. Soc. 1980,102,5565-5572.(b) Girault, J. P.;Chottard, G.;Lallemand, J. Y.; Chottard, J. C. Biochemistry 1982, 21,1352-1356. (c) Dewan, J. C. J. Am. Chem. Soc. 1984,106,7239. (d) Fouts, C. S.;Marzilli, L. G.;Byrd, R. A.; Summers, M. F.; Zon, G.;Shinozuka, K. Inorg. Chem. 1988,27,366-376. (e) Caradonna, J.; Lippard, S.J. Inorg. Chem. 1988,27, 1454-1466. (f) Reily, M. D.; Hambly, T. W.; Marzilli, L. G. J. Am. Chem. Soc. 1988, 110, 2999-3007. (g) Martin, R. B. ACS Symp. Ser. 1983, No. 209, 231-244. (h) van der Veer, J. L.; van den Elst, H.; den Hartog, J. H. J.; Fichtinger-Schepman,A. M. J.; Reedijk, J. Inorg. Chem. 1987,26, 1536-1540. (i) Inagaki, K.; Kidani, Y . Inorg. Chem. 1986, 25, 1-3. (4) (a) Marcelis, A. T. M.; van Kralingen, C. C.; Reedijk, J. J. Inorg. Biochem. 1980,13,213. (b) Inagaki, K.; Dijt, F. J.; Lempers, E. L. M.; Reedijk, J. Inorg. Chem. 1988. 27, 382. (c) Hemelryck, B. V.; Girault, J. P.; Chottard, G.;Valadon, P.; Laoui, A.; Chottard, J. C. Inorg. Chem. 1987.26,787-795. (d) Laoui, A.; Kozelka, J.; Chottard, J. C. Inorg. Chem. 1988,27, 2751-2753. ( 5 ) (a) Van der Veer, J. L.; Noteborn, H. P. J. M.; Van der Elst, H.; Reedijk, J. Inorg. Chim. Acta 1987,131,221. (b) Lempers, E. L. M.; Inagaki, K.; Reedijk, J. In Platinum and other metal coordination compounds in cancer chemotherapy;Nicolini, M., Ed.; Nijhoff: Boston, 1918, 1988;pp 110-1 14.
+ +
where k, = k,, + kzc kk kb. Each kinetic constant has been calculated from the relative peak areas of HPLC chromatograms. The kinetic parameters ( k , and k,) are in fair agreement with the corresponding parameters reported by Chottard et aL8 for the reaction between [cis-Pt(NH,)2(OH2)2]2+ and r (GpA) . In the Pt adducts 2-4, the binding ratio of [Pt(R,R-dach)12+ to r(GpA) was found to be 1:1, Le., Pt(R,R-dach)(r(GpA)), being calculated from integrations of the H1' protons of the r(GpA) moiety and the C H protons of the cyclohexane ring in the NMR spectra. Adduct 3 can definitely be assigned to Pt(R,R-dach)(r(GpA)-N7(1),N7(2)) from evidence of UV, CD (data not ~~
~
(6) The Pt adducts are called 1-4 according to their HPLC elution order.
(7) The relative ratios were calculated from peak areas of the HPLC chromatogram. Although the molar extinction coeflicients (at 260 nm) of the Pt adducts and unreacted r(GpA) are not identical, they are expected to be almost the same (Inagaki, K.; Tomita, A.; Kidani, Y. Bull. Chem. SOC.Jpn. 1988,61,2825). (8) These authors" followed the reaction between r(GpA) and [cis-Pt-
(NH3)2(0H2)2]2+by means of UV spectrometry and HPLC and obtained the values k, = 0.8 M-l s-I and k, = 4.5X IO4 s-I at 20 O C , pH = 5.2. They isolated ci~-Pt(NH~)~(r(GpA)-N7(l),N7(2)) (68%) and ci~-Pt(NH,)~(r(GpA)-N7(l),N1(2)) (32%). and the latter compound corresponds to the adduct 4 in this work.
0020-1669/90/ 1329-2183$02.50/0 0 1990 American Chemical Society
2184 Inorganic Chemistry, Vol. 29, No. 11, 1990
Notes
Table I. Products Obtained from the Reaction between Pt(R,R-dach)(OH,), and r(GpA) and Kinetic Constants for the Reaction
abbr
comd ~~~
~
r(Gp.4)
Pt(R,R-dach)(OH2)(r(GpA)-N7(1))
int 1
d Pt( R,R-dach)(r(GpA)-N7( I ) , N I ( 2 ) ) Pt( R,R-dach)(r(GpA)-N7( I),N7( 2 ) )
2
3 4
Pt(R,R-dach)(r(GpA)-N7(I),N1(2))
retention time." min 19.4 15.8 6.9 9.6 11.1 14.0
R ratio of 1-4.' %
rate constC (at IO OC)
kl = (0.67 f 0.07) M-' s-I (2.2 0.3) x 10-5 s-I
k,, = k2, = k,, = kk =
3.3-3.6 6.8-7.5 36-39 50-54
*
(4.1 i 0.5) X s-I (2.2 i 0.3) X IO4 s-I (3.2 i 0.4) X IO4 s-I
"The parameters of the HPLC run are as follows: column, Cosmosil 5Cls (0.46 cm i.d. X 15 cm); detector, UV at 260 nm; flow rate, 1.2 mL/min; mobil phase, linear gradient elution (I%/min) from 0.05 M KH2P0, (pH 4.6) to methanol. bValues obtained for the first IO h of reaction at 37 OC. ' k , , , k2,, k,,, and kg are kinetic constants for the formation reactions of 1-4, respectively, from the aqua intermediate. dCompound not definitely determined. From pH-UV and pH-NMR titrations, the pK, value of the G-NI site of Pt adduct 1 is found to be 8.2. N o protonation at G - N 7 and A-N 1 is observed.
shown), and NMR spectral data and pKa values of the G-NI (pK, = 8.2) and A-NI sites (pK, = 1.5) after platination." It contains two kinds of rotamers, likely G,,ti-A,,,i(75%)9a and Ganti-ASy,(25%),9b because two sets of three protons are observed in the purine base proton region. Adduct 2 can definitely be assigned to Pt(RJ?-dach)(r(CpA)-N7(I),N1(2)). This adduct also contains two rotamers,Ih most likely Ganli-Aanti(42%)Iob and Ganti-Aryn (58%).'OE From UV, CD, and NMR spectral evidence, it appears that adduct 4 has the same structure as c i ~ - p t ( N H ~ ) ~ ( r ( G p A ) N7(1),N1(2)), being reported by Chottard et al." They assigned c ~ s - P ~ ( N H ~ ) ~ ( ~ ( GI),N1(2)) ~ A ) - N ~to( G,-A, with a rotation of the adenine base about the glycosidic and Pt-A-NI bonds. Figure 1 shows a pH titration curve of the adduct 4. No protonation at the G-N7 and A-NI sites is observed, and deprotonation at G-N 1 affects not only the chemical shift of G-H8 but also those of A-H2 an A-H8. This suggests that the deprotonation of G-N 1 induces a certain change in conformation of the A base. The G-H8 and A-H8 signals broaden very much at alkaline pH region (see Figure These NMR spectral features are in good agreement with Chottard's data.8 We therefore assign 4 to an interbase cross-linked compound with G-N7 and A-NI, probably Pt(R,R-dach)(r(GpA)-N7(1),Nl(2)) with Gsfl-A,. The adduct 1, unfortunately, could not be sufficiently characterized because of the very small amount. As described above, the relative ratio of the four platinum adducts was almost constant for the first 10 h (at 37 "C). But the subsequent incubation results in a decrease of 4 and an increase of 1-3 even after disappearance of free r(GpA) and the aqua 1:l intermediate. This strongly points toward an isomerization from 4 to 1-3. Figure 2 shows a time dependence of the conversion reaction from 4 to 1-3. The half-life of 4 was 8.8 h at 60 "C. The relative ratio of the end products 1-3 was constant throughout the isomerization reaction and also similar to the values obtained from the reaction of [Pt(R,R-da~h)(oH,)~]~+ with r(GpA). The adduct 4 decreases according to the first-order reaction. Although the adducts 1-3 are formed without an observable intermediate, they are likely to be formed via the aqua intermediate, Pt(R,Rdach)(OH,)(r(GpA)-N7(1)),Le., according to the following scheme: 4
k k-8
aqua intermediate
k,
1-3
(2)
The reaction scheme corresponds to the formation reaction of the four Pt adducts from the aqua intermediate in eq I . If solvolysis reaction of 4 is very slow, i.e. k, >> k, or k , + k,, >> k,, the (9) (a) NMR (ppm): A-H8, 9.34; G-H8, 8.55; A-H2, 8.34. The T1 relaxation times of A-H8 and G-H8 are 1 .05 and 0.75 s-I at 26 "C and 400 MHz, respectively. (b) NMR (ppm): A-H8, 9.14; G-H8, 8.54; A-H2, 8.30. The TI relaxation times of A-H8 and G-H8 are 1.30 and 0.54 s-I at 26 OC, respectively. (10) (a) Chemical shift data for 2 are in good agreement with those of cis-Pt(NH,),(d(GpA)-N7(I),NI(2)) with Ganti-Alyn (30%) and Ganli-A,.,i (70%): Dijt, F. J.; Chottard, J. C.; Girault, J. P.; Reedijk, J. Eur. J . Biochem. 1989, 179, 333. The data for cis-Pt(NH,),(d(GpA)-N7(1),Nl(2)) are indicated within parentheses. (b) NMR (ppm): A-H8.8.34 (8.40); G-H8, 7.92 (7.91); A-H2.8.43 (8.48). (c) NMR (ppm): A-H8,8.26 (8.25); G-H8,8.64 (8.61); A-H2,8.68 (8.61). ( 1 1) The G-HI' signal also broaden from 4 (pH 2-6) to 33 Hz (pH 9.3).
/I
I 0.00
140
280
420
560
l n c u b a t i o n t i me
700 ( m i n. 1
Figure 2. Time dependence of the isomerization reaction of the Pt adduct 4 at 6 0 OC in water. [4] = 3.0 X IO4; pH = 5.5.
apparent rate constant, kobe(,),in reaction scheme 2 is given by kob+, = k,k,/(k, + k,), where k , = k l , + klc + kScand k , = kk, The following values were calculated from the data in Table I and from the valuesI2 of kobs(s)at 37,61, and 77 OC: kobs(s)= 9.7 X s-I, k, = 2.0 X IO-'s-l, and k , = 2.8 X IO4 s-l at 10 O C . The kinetic parameters (supplementary material Table SI) already indicate that the aqua intermediate is difficult to be detected because k , k , >> k,. Moreover, thermodynamic parameters calculated from an Eyring plot suggest that the isomerization reaction proceeds via an association mechanism (AH' = 78.5 kJ mol-l and AS' = -103 J K-' The aquation path in the reaction appears to be supported by the decrease in rate when the solvent is substituted for a water-ethanol mixture (the values of kobs(s)(X105 s-l) at 60 OC are 2.2, 1.4, and 1.0 at 0%, 25%, and 50% ethanol, respectively). The rate of the isomerization reaction tends to be slightly speeded up with lowering the pH (the values of kobs(s)at 37 O C (X106 s-I) are 3.4, 2.7, 2.2, 1.7, and 0.7 at different pHs 1.8, 2.5, 4.4, 6.2, and 9.1, respectively). The relative ratio of the final Pt adducts 1-3-being obtained from the isomerization reaction-also changes as a function of pH. The formation of 1 and 2 tends to be suppressed with lowering the pH. Especially the formation of 2 is strongly
+
(12) The Kob(a)value at 10 'C was estimated from extrapolation of k*(,) vs temperature because the conversion reaction of 4 to 1-3 was too slow to measure at low temperature and an Eyring plot using apparent kinetic constant (k& *) showed a clear straight line (the values of kw,are 1.5 X l e , 1.5 X fV5,and 5.7 X IC5s-I at 37,61, and 77 OC (pH 6);solvent 9% methanol), respectively). (1 3) (a) Basolo, F.;Pearson, R. G. Mechanism of Inorganic Reactions, 2nd ed.; Wiley: New York, 1967; pp 351-453. (b) Girault, J. P.; Chottard, G.; Lallemand, J. Y.;Huguenin, F.;Chottard, J. C. J . Am. Chem. Soc. 1984, 106, 7227-7232.
2185
Inorg. Chem. 1990, 29, 2185-2188 suppressed at low pH (supplementary material Figure Sl). This is likely to be due to the fact that protonation at A-Nl of the aqua intermediate inhibits the formation of 2. The competitions at A-N 1 between platinum and proton are also expected to occur in the reaction forming 4 from the aqua intermediate, i.e. k,, and this is likely to lead a promotion of kobs(,)observed at low pH. The isomerization from 4 to 1-3 is enhanced in a presence of halogen ions. In the HPLC chromatogram, an intermediate peak, presumably Pt(R,R-dach)(X)(r(GpA)-N7(1)), X = CI,Br, and I, has been observed. The observed rate constant for the conversion of 4 shows a linear dependence upon an excess of halogen ion concentration ([XI= 0.01-0.2 M) and a nonzero intercept at [XI = 0. The data fit well in a two-term rate expression (kob = kOF(,) k x [ X ] ) ,which is the well-known rate law for the substitution reactions of the square-planar platinum complex. The rate constants obtained from the intercepts were found to agree within experimental error with the rate constant obtained in an absence of halogen ions. The result also supports the solvolytic path in the isomerization reaction. The values14 of k x increases in the order CI- < Br- < I-. In conclusion, the chelate formation reaction between [Pt(R,R-dach)lZ+and r(GpA) proceeds according to the following reaction scheme (in the absence of halogen ions).
+
r(GpA)
-
+ pt(R,R-da~h)(OH,)~
kl
aqua intermediate k, (slow) k, (fast)
4'
aqua intermediate
k,
fast
kc
1-4
1-3
The reverse reaction (from 1-3 to 4) was not observed. It is worth noting that the same reaction scheme has also been observed in the reaction between C ~ ~ - [ P ~ ( N H ~ ) ~ (and O Hr(GpA) ~ ) ~ ] and ~+ d(GpA) and that the conversion reaction such as 4 1-3 could not be observed in the case of reaction between [Pt(R,Rda~h)(OH,),]~'and r(ApG) (unpublished observation). Acknowledgment. This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science and Culture and from the Ministry of Health and Welfare of Japan. Prof. Dr.J. C. Chottard (University of Paris) is thanked for critical reading and valuable comments about the manuscript.
-
Supplementary Material Available: Table SI, giving kinetic data for the isomerization reaction, and Figure SI,showing the relative ratio of the final Pt adducts, 1-3, as a function of pH (2 pages). Ordering information is given on any current masthead page.
M-ls-l, kBr= 4.0 X lO-'M-l s-I, and k, = 8.1 X (14) k,-l,=$.7 X M- s at 37 OC.
Contribution from the Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214
Preparation and Characterization of trans-Bis(a-dioximat0)ruthenium Complexes James G. Muller a n d Kenneth J. Takeuchi* Received October 6 , I989
The synthesis and characterization of transition-metal complexes that contain a ruthenium metal center coordinated to chelating nitrogen donor ligands have been subjects of considerable interest - ~addition, ~ the coordiin our laboratory1" and e l ~ e w h e r e . ~In ( 1 ) Muller, J. G.;Takeuchi, K. J. Inorg. Chem. 1987, 26, 3634. (2) Leising, R. A.; Takeuchi, K. J. Inorg. Chem. 1987, 26, 4391. (3) Marmion. M. E.; Kubow, S. A,; Takeuchi, K. J. Inorg. Chem. 1988,27, 2761.
A
R u ( N O ) C I ~ * ~ H+~2.1 O LH~T
LHp=
DMGH, ,
DFGH,
,
Ru(LH)~(NO)CI
N o w ,
DPGH2
Figure 1. Reaction scheme for the synthesis of trans-bis(a-dioximato)ruthenium complexes. DMGM, = dimethylglyoxime, DFGH, = a-furil dioxime, NOXH, = 1,2-cyclohexanedione dioxime, and DPGH, = diphen ylgl yoxime.
nation chemistry of chelating a-dioxime ligands has been extensively investigated with first-row transition metals,22-26where trans-bis(a-dioximato) transition-metal complexes have been utilized as analytical models for biological systems such as vitamin B12,30-32dioxygen carrier^,)^"^ and catalysts in chemical p r o c e ~ s e s . ~ ~ The - ~ * only examples of rrans-bis(a-di-
Marmion, M. E.; Leising, R. A.; Takeuchi, K. J. J . Coord. Chem. 1988, 19, I . Marmion, M. E.; Takeuchi, K. J. J . Am. Chem. Soc. 1988,110, 1472. Leising, R. A.; Kubow, S.A.; Churchill, M. R.; Buttrey, L. A.; Ziller, J. W.; Takeuchi, K. J. Submitted for publication. Thummel, R. P.; Lefoulon, F.; Chirayil, S.Inorg. Chem. 1987, 26, 3072. Constable, E . C.; Seddon, K. R. J . Chem. SOC.,Chem. Commun. 1982, 34. Goswami, S.;Chakravarty, A. R.; Chakravorty, A. Inorg. Chem. 1981, 20, 2246. Chang, J.; Meyerhoffer, S.; Allen, L. R.; Durham, B.; Walsh, J. L. Inorg. Chem. 1988, 27, 1602. Barqawi, K. R.; Llobet, A.; Meyer, T. J. J . Am. Chem. Soc. 1988,110, 7751. Taqui Kahn, M. M.; Sreelatha, Ch.; Mirza, S.A.; Ramachandraiah, G.; Abdi, S. H. R. Inorg. Chim. Acta 1988, 154, 103. Bao, T.; Krause, K.; Krause, R. A. Inorg. Chem. 1988, 27, 759. Che, C. M.; Tang, W. T.; Lam, M. H. W.; Mak, T. C. W. J . Chem. SOC.,Dalton Trans. 1988, 2885. Charalambous, J.; Stoten, W. C.; Henrick, K. Polyhedron 1989,8, 103. Kurtev. K.; Ribola, D.; Jones, R.; Cole-Hamilton, D. J.; Wilkinson, G. J. Chem. SOC.,Dalton Trans. 1980, 5 5 . Warren, L. F.; Goedken, V. L. J . Chem. SOC.,Chem. Commun. 1978, 909. Walker, D. D.; Taube, H. Inorg. Chem. 1981, 20, 2828. DeCola, L.; Barigelletti, F.; Balzani, V.; Belser, P.; von Zelewsky, A,; Vogtle, F.; Ebmeyer, F.; Grammenudi, S.J . Am. Chem. Soc. 1988, 110, 7210. Chakravorty, A. Coord. Chem. Rev. 1974, 13, I . Banks, C. V. Rec. Chem. Prog. 1964, 25, 85. Schrauzer, G. N. Chem. Ber. 1962, 95, 1438. Caton, J. E.; Banks, C. V. Talanra 1966, 13, 967. Stynes, D. V.; Pang, I. Inorg. Chem. 1977, 16, 590. Pomposo, F.; Stynes, D. V. Inorg. Chem. 1983, 22, 569. Bakac, A.; Brynildson, M. E.; Espenson,J. H. Inorg. Chem. 1986, 25, 4108. Middleton, A. R.; Thornback, J. R.; Wilkinson, G. J . Chem. Soc., Dalton Trans. 1980, 174. Fukuchi, T.; Miki, E.; Mizumachi, K.; Isimori, T. Chem. Lett. 1987, 1133. Singh, R. B.; Garg, B. S.; Singh, R. P. Talanta 1979, 26, 425. Schrauzer, G . N. Acc. Chem. Res. 1968, I, 97. Schrauzer, G. N.; Sibert, J. W. J. Am. Chem. SOC.1970, 92, 7078. Toscano, P. J.; Seligson, A. L.; Curran, M. T.; Skrobutt, A. T.; Sonnenberger, D. C. Inorg. Chem. 1989, 28, 166. Schrauzer, G. N.; Sibert, J. W. J . Am. Chem. SOC.1970, 92, 1551. Tovrog, B. S.; Kitko, D. J.; Drago, R.S. J . Am. Chem. Soc. 1976,98, 5144. Rochinbauer, A,; Eyor, M.; Kwiecinski, M.; Tyrlik, S. Inorg. Chim. Acta 1982, 58, 237. Noguchi, M.; Kambara, S. J. Polym. Sci., Part B 1963, 553.
0020~1669/90/1329-2185$02.50/0 0 1990 American Chemical Society
